STUDY OF SURFACE TREATMENTS AND PHOSPHATATION OF GALVANISED INDUSTRIAL PRODUCTS IN VIEW OF THEIR IMPROVED CORROSION RESISTANCE

Abstract

Surface properties of various coatings have been studied, as well as their aptitude to phosphatation, paint adhesion and corrosion resistance. The coatings used for this study were electroplated pure Zn, Zn-8%Ni, Zn-3%Fe and hot-dipped pure Zn (minimised spangle galvanised), Zn-10 to 15%Fe (galvannealed), brushed Zn (monogal), Zn-5%Al (galfan) or Zn-55%Al (galvalume). The properties of the layers formed after hot-dip galvanisation have received special attention. Various phases and intermetallic compounds have been identified, in particular ZnAl(2)O(4) at the outer surface and Fe(2)A1(5) with Zn in substitutional positions, at the Zn-Fe interface of the galvanised spangle. After bending tests, cracks are observed in all coatings except galfan, which is unaffected; however all coatings remained adherent. Even though the phosphatation kinetics of the various coatings were quite different, after completion of the phosphatation treatment the layers' properties were quite similar, i.e., weight, thickness, alkaline agents resistance, surface reactivity. An alkaline pretreatment favours a good phosphatation. Electrochemical measurements show that the coatings studied here provide good cathodic protection to the substrate, the important parameter being the thickness of the coating rather than its nature.